Meet the Diprotodon's ancestor, the original big flat-footed marsupial
Scientists have identified an ancient marsupial for the first time, whose special adaptations allowed it to walk great distances across the continent now known as Australia some 3.5 million years ago. For an animal that would have weighed more than a quarter of a tonne, that trekking talent was no mean feat. (Or feet, should we say?)
Using 3D scanning to assemble an eroded skeleton found in 2017 in South Australia, paleontologists from Australia’s Flinders University have for the first time recreated this previously unknown ancestral marsupial, newly classed as Ambulator keanei, the first of its kind in the novel genus Ambulator.
Taxonomy classification has generally done a decent job with organism names, so it’s fitting that this large quadruped has been crowned Ambulator, meaning walker or wanderer. The scientists believed it adapted to a changing landscape as the Australian continent went from lush forest to open woodland and shrubland during the Pilocene age. How? With flat feet (plantigrady) assisted by developing a secondary heel in their hands.
Plantigrade foot structure enables good support and stability, which can be seen in animals that evolved to walking comfortably on two legs (like humans). However, the sacrifice is speed. This suggests the herbivorous A. keanei needed to cover larger distances more than it did fleeing any predation threats. In some species, such as bears, their flat feet help with stability when fighting, but there’s little known about what these massive meandering marsupials might have needed that for, despite crocodiles and lions predators among the megafauna of the time.
However, it seems it was clearly an important enough evolutionary trait, as the animal’s more famous, larger Diprotodontidae family descendent, Diprotodon optatum, also retained this function. It’s still a feature of marsupials today (as well as rodents, rabbits and birds), including perhaps these extinct animals’ most closely related species, wombats.
“Most large herbivores today such as elephants and rhinoceroses are digitigrade, meaning they walk on the tips of their toes with their heel not touching the ground,” said Jacob van Zoelen, the study’s first author and student at Flinders University.
“Diprotodontids are what we call plantigrade, meaning their heel-bone (calcaneus) contacts the ground when they walk, similar to what humans do,” he added. “This stance helps distribute weight when walking but uses more energy for other activities such as running.”
While a lot of fossil study focuses on dentition and other areas of the skull, this discovery highlights how much can be learned about an ancient animal and how it got around through its feet.
“We don’t often think of walking as a special skill but when you’re big any movement can be energetically costly so efficiency is key,” said Van Zoelen. “Development of the wrist and ankle for weight-bearing meant that the digits became essentially functionless and likely did not make contact with the ground while walking. This may be why no finger or toe impressions are observed in the trackways of diprotodontids.
“So, diprotodontids such as Ambulator may have evolved this morphology to traverse great distances more efficiently. This morphology also allowed for greater weight to be supported, allowing diprotodontids to get very big indeed.
“Eventually, this led to the evolution of the giant and relatively well-known Diprotodon.”
The research was published in the Journal of Royal Society Open Science.
Source: Flinders University